The present invention relates to methods for the measurement of surface polarity of inorganic materials. In particular, the present invention relates to a method for the measurement of the surface hydrophobicity or hydrophilicity of inorganic material, particularly materials which have been surface treated.
Inorganic materials in particulate form, such as silica (SiO.sub.2) and titania (TiO.sub.2), are used as thixotropic agents and as fillers in materials as diverse as inks, resins, rubbers, circuit boards, paints, and cosmetics. An important use of silica and other inorganic powders is as fillers, i.e., as reinforcing agents for polymer matrices to form composites. Composites comprising particulate silica and silicone elastomers have found particular utility in household goods such as sealants, or in automotive and oven gaskets, electrical insulators and connectors, baby bottle nipples, and the like.
The reinforcing properties of fillers, and fumed silica in particular, are strongly dependent on their morphology and surface properties. Morphology is determined primarily by the synthetic method used to obtain the filler, whereas surface properties are determined primarily by surface polar groups, which render the fillers hydrophilic. In silica these polar groups are typically hydroxyl groups, most often present as silanol groups. To be effective as a filler in polymer systems, the filler surface may be treated to change its polarity. Typically, the fillers are rendered hydrophobic by such treatment. Typical methods for rendering silica hydrophobic are disclosed in U.S. Pat. Nos. 4,554,147 and 5,652,017 to Osaheni et al., wherein a silica powder is treated with a silylating agent comprising, for example, a polyorganosiloxane, triorganosilane, or diorganosilane. Differences in the number of surface polar groups, particularly surface hydroxyls, and/or the nature of hydrophobic groups on the surface of the filler after treatment will result in the filler having different characteristics.
In many applications it is critical to determine the degree of hydrophobicity of the powder fillers, in particular silica fillers. It is particularly important to ensure that treated fillers are consistent from batch to batch, in order to guarantee the properties and quality of the composite product.
Treated inorganic powders may be characterized by a series of tests, including thermogravimetric analysis combined with Fourier transform infrared analysis (TGA-FTIR) to determine the amount of physically absorbed species; carbon analysis; proton nuclear magnetic resonance (.sup.1 H NMR) spectroscopy; and silicon-29 cross polarization and magic angle spinning nuclear magnetic resonance (.sup.29 Si CP/MAS NMR) spectroscopy. Acid-base titrations can also be used, but are inaccurate where the residual titratable groups, such as silanols, are not readily accessible. Depending on the silylating agent, at least two or three of these analyses need to be performed in order to determine the hydrophobic nature of the inorganic powder. Such extensive analyses are expensive and time consuming. Accordingly, there remains a need in the art for accurate and efficient methods for measuring the surface polarity, in particular the surface hydrophobicity of inorganic materials, particulates, and powders.